The invention relates to a method of reading data from a disc carrier and applying the data to an output comprising the steps of;
i) reading data from the disc and writing it into a buffer memory, PA1 ii) reading the data stored in the buffer memory to the output at a first given data rate, PA1 iii) detecting any interruption in the reading of data from the disc to the buffer memory, and PA1 iv) reading data from the disc at a second given data rate which is higher than the first given data rate after an interruption in reading data from the disc to the buffer memory, in order to restore the quantity of data in the buffer memory to a given level to thereby enable uninterrupted reading of data to the output in the presence of shocks which cause a temporary interruption of the reading of data from the disc to the buffer memory. PA1 v) reading data from a position on the disc subsequent to the point at which the shock occurred, and writing it into the buffer memory, as soon as it becomes available after an interruption, and PA1 vi) reading data from the buffer memory to the output in the same order as it is stored on the disc carrier. PA1 vii) interrupting the writing of data to the buffer memory when the quantity of data in the buffer memory reaches a first level where the buffer memory is substantially full, and PA1 viii) resuming the writing of data into the buffer memory when the quantity of data in the buffer memory falls to a second lower level.
The invention further relates to an arrangement for reading data from a rotating disc carrier, said arrangement including a shock compensation memory for providing output data when a reading head becomes misaligned with respect to information tracks on the disc. The said arrangement also comprises means for realigning the reading head at a later location on the track, means for detecting when the disc has rotated to a position where the reading head is located radially aligned with but outside the position at which misalignment occurred, means for causing the reading head to move inward one or more track(s) so that it regains the position at which the misalignment occurred, and means for causing data to be written into the memory at a rate greater than that at which it is being read out until the memory becomes substantially full.
Such a method and arrangement has been disclosed, inter alia, in U.S. Pat. No. 4,796,247, EP-A-0 429 130, EP-A-0 550 097, WO91/11002 and WO91/11003. In all these documents a buffer memory is provided so that a continuous output of data can be obtained when mechanical shocks occur which cause the reading head to become temporarily misaligned with the data track on the disc. Thus by reading out the data from the buffer memory the effect of a limited duration shock on the output can be eliminated, i.e as far as a listener to a compact disc player is concerned the shock has no effect on the output.
In order to achieve this result data is read from the disc and written into the buffer memory at a higher rate, for example twice, than that at which it is read out from the buffer memory. It is possible to read the data from the disc continuously at the higher rate, to interrupt this reading when the buffer memory is about to overflow, and restart the reading when the buffer memory empties to a predetermined state, as disclosed in EP-A-0 429 139. Alternatively, the data may normally be read from the disc at the same rate as it is read from the buffer memory, the buffer memory being initially filled before output begins and then in the case of the buffer emptying, because of an interruption of reading data from the disc due to head misalignment caused by a mechanical shock, the rate of reading of the data is increased (doubled) until the buffer memory is re-filled. Once the buffer memory is full the rate of reading the data from the disc is restored to the original rate. This has the disadvantage, however, that the speed of rotation of the disc has to be changed.
The duration of a shock which can be compensated by such an arrangement is, of course, limited by the capacity of the buffer memory, i.e the maximum length shock which can be compensated corresponds to the time taken to read out all the data in the buffer memory. Additionally, a series of shorter shocks may also exceed the memory capacity if there is insufficient time between the shocks to re-fill the buffer memory, i.e as the frequency and number of shocks increase the buffer memory is progressively emptied. Successive shocks may occur, for example when the arrangement is used in a personal CD player which may be worn by the user whilst jogging or taking part in other physical activities. Another circumstance which can give rise to successive shocks is use in a car or other vehicle.
The above citations are hereby incorporated in whole by reference.